In an active matrix type display, typically an active matrix type liquid crystal display, a thin film transistor (hereinafter abbreviated as TFT) is formed in each pixel, and display information is stored on a pixel-by-pixel basis to display images. A TFT formed by using a polysilicon film which is fabricated by polycrystallization of an amorphous silicon film by laser annealing, with its mobility being raised to about 100 cm2/VS is called a polysilicon TFT. Since a circuit configured of such polysilicon TFTs operates with signals of a few MHz to dozens of MHz at the maximum, not only pixels but also a data driver circuit generating image signals and a scanning circuit can be formed over the substrate of a liquid crystal display device or the like in the same process as the formation of the TFTs constituting pixel circuits.
A transmissive liquid crystal display performs display by controlling the transmittance of transmitted light of a backlight. On the other hand, a reflective liquid crystal display which has a reflecting electrode for reflecting external light in a pixel performs display by controlling the reflectance of sunlight or room illumination light that comes in pixels, thereby negating the need for a backlight.
Further, a liquid crystal display having both the functions of transmission and reflection is called a semi-transmissive liquid crystal display. In general, the reflective liquid crystal display and the semi-transmissive liquid crystal display in a state where the backlight is not lit feature much lower power consumption compared to the transmissive liquid crystal display which requires the backlight to light up.
Liquid crystal displays enhancing such a low power consumption feature include a liquid crystal display with built-in pixel memory. Since an ordinary liquid crystal display without built-in pixel memory temporarily stores electric charge in a capacitor in a pixel to hold voltage that is applied to the liquid crystal, it is necessary to refresh the voltage at regular time intervals even in the case of displaying a static image. Thus, in either case of displaying a moving image or a static image, data lines for transferring data signals to pixels needs to be driven at about dozens of kHz; therefore, the data lines and the data driver circuit for driving the data lines consume much power.
The liquid crystal display with built-in pixel memory which places emphasis on displaying static images incorporate a static memory in each pixel, thereby negating the need for refresh operation and therefore making it possible to completely cut power consumed by the data lines and the data driver circuit.
FIG. 9 shows the configuration of a conventional display with built-in memory. Pixel circuits 82 are arranged in a matrix form over a glass substrate 81.
In FIG. 9, the pixel circuits 82 are arranged only in two columns by three rows, for simplicity of explanation. However, the actual numbers of columns and rows are both over several hundreds. A pixel circuit 82 is composed of a sampling TFT 83 for sampling data from a data line, a static memory 84 for storing 1 bit of data, and an AC circuit 85 for applying AC voltage corresponding to the storing state of the static memory 84 to a liquid crystal LC as a display section.
Each pixel circuit 82 is connected to data lines s1 to s2 and gate lines g1 to g3 through the sampling TFT 83. The data lines s1 to s2 are connected to a data driver circuit 86, and the gate lines g1 to g3 are connected to a scanning circuit 87. The data driver circuit 86 has the function of temporarily storing video signals serially inputted from the outside of the display and parallelly outputting to the data lines s1 to s2.
The scanning circuit 87 sequentially outputs pulses to the gate lines g1 to g3 in synchronization with the output operation of the data driver circuit 86, thereby determining a horizontal row of pixel circuits 82 for writing a video signal generated on the data lines s1 to s2. The sampling TFT 83 is turned on by a pulse supplied to the connected gate line, thereby writing the signal of the connected data line into the static memory 84.
The AC circuit 85 selects a square wave voltage VLCa or VLCb in accordance with the state of 1-bit data stored in the static memory. The voltage Vcom is a square wave voltage having a frequency of about 30 to 60 Hz, the voltage VLCa is a square wave voltage in phase with Vcom, and the voltage VLCb is a square wave voltage of opposite phase to Vcom. For example, assume that a normally white liquid crystal (in which bright display is performed when the applied AC voltage is small in amplitude) and an optical structure required therefor are employed, for example. When the voltage VLCa is selected, in-phase signals are applied to the liquid crystal LC; therefore, the applied AC voltage becomes low and the liquid crystal cell LC displays white. On the other hand, when the voltage VLCb is selected, opposite-phase signals are applied to the liquid crystal LC; therefore, the applied AC voltage becomes high and the liquid crystal cell LC displays black. The liquid crystal display device with built-in memory is described in more detail in JP-A-8-194205 (194205/1996) and JP-A-8-286170 (286170/1996).
In accordance with the state of 1-bit data stored in the static memory 84, the white display or black display of each pixel can be selected. Accordingly, in the case where video data is not rewritten, it is possible to display a static image even if the operation of the data driver circuit 86 and the scanning circuit 87 is stopped. Since this makes it possible to cut all the power for driving the data lines s1 to s2 and the gate lines g1 to g3, the display with built-in memory can reduce power consumption during static image display, compared to an ordinary liquid crystal display.